The need to define antigenic differences among human rotaviruses led us to cultivate directly in cell culture a variety of rotavirus strains from diverse geographical areas and populations. Selected rotavirus strains are being developed as potential vaccine candidates. Over 90 strains of human rotaviruses have been cultivated in MA104 or African green monkey kidney (AGMK) cells. Four distinct serotypes of human rotavirus were identified and compared with each other by plaque reduction or tube neutralization assay. Based upon these findings a numbering system which includes both human and animal rotaviruses was proposed.